Glutamine is a "conditionally essential amino acid": Although, glutamine is considered a non-essential amino acid, recent reports and studies from our laboratory support that the presence of intraluminal (apical) glutamine is essential for optimal intestinal epithelial function (Horvath K, Jami M M, Hill I D, Papadimitriou J C, Magder L S, Chanasongcram S. Glutamine-free oral diet and the morphology and function of the rat small intestine, J Parent Ent Nutr 1996;20: 128-134).
Small intestinal epithelial cells (enterocytes) have the third highest rate of cell turnover in the body, and require a constant high energy source. Glutamine is the preferred fuel for enterocytes (Williamson R C N. Intestinal adaptation. Structural, functional and cytokinetic changes, N Engl J Med 1978; 298:1393-1402; Nygaard K. Resection of the small intestine in rats. 3. Morphological changes in the intestinal tract. Acta Chir Scand 1967; 133: 233-248; Windmueller H G., Spaeth A E.: Uptake and metabolism of plasma glutamine by the small intestine. J. Biol. Chem. 1974; 249: 5070-5079).
The enterocytes are strongly dependent on an external glutamine supply, because of the small size of the mucosal glutamine pool in the small intestine (0.2 .mu.mol/g of tissue) compared to liver (5.94 .mu.mol/g) or skeletal muscle (3,3 .mu.mol/g). The activity of mucosal glutamine synthetase is also extremely low (Windmueller H G., Spaeth A E.: Uptake and metabolism of plasma glutamine by the small intestine. J. Biol. Chem. 1974; 249: 5070-5079; Lund P.: A radiochemical assay for glutamine synthetase, and activity of the enzyme in the rat tissues. Biochem. J. 1970; 118:35-39).
Although enterocytes can get glutamine from both the lumen and circulating blood, these two sources of glutamine are not utilized in identical manner. During total parenteral nutrition (TPN) without glutamine, intestinal atrophy and hypofunction occurs within 3 days, despite the fact that L-glutamine levels in serum do not decrease significantly. Although L-glutamine is not regarded as an essential amino acid, its necessity during TPN makes it a conditionally essential amino acid in humans (Hughes C A, Dowling R H. Speed of onset of adaptive mucosal hypoplasia and hypofunction in the intestine of parenterally fed rats. Clin Sci 1980; 59:317-327).
Atrophy of the intestine also involves cells and organelles other than enterocyte. Immunological cells, such as lymphocytes and macrophages, in the mucosa also metabolize glutamine. Fibroblasts require glutamine for optimal function, as well (Ardawi M S M, Newsholme E A. Glutamine metabolism in lymphocytes of the rat. Biochem J. 1983; 212: 835-842; Caldwell M D. Local glutamine metabolism in wound and inflammation. Metabolism 1989; 38(suppl):34-39; Zielke H R, Ozand P T, Tildon J T, Sevdalian D A, Cornblath M. Reciprocal regulation of glucose and glutamine utilization by cultured human diploid fibroblasts. J Cell Physiol 1978; 95:41-48).
In the absence of glutamine the rate of (.sup.3 H) thymidine incorporation into DNA was very low in mesenteric lymphocytes and even a small amount of glutamine (1 .mu.M) caused a four-fold increase in incorporation. TPN formulas without glutamine result in significant decrease in the secretory IgA level associated with bacterial translocation from the gut. 2% glutamine in TPN significantly decreased the bacterial (E. coli, Proteus mirabilis) translocation to mesenteric lymph nodes in rats (Burke D J, Alverdy J C, Aoys E, Moss G. Glutamine-supplemented total parenteral nutrition improves gut immune function. Arch Surg 1989; 124:1396-1399), although there was no significant difference in the adherence of bacteria to the ileum and colon (Ardawi M S M, Newsholme E A. Glutamine metabolism in lymphocytes of the rat. Biochem J. 1983; 212: 835-842; Alverdy J C, Chi H S, Sheldon G S. The effect of parenteral nutrition on gastrointestinal immunity, the importance of enteral stimulation. Ann. Surg. 1985; 202:681-684; Alverdy J C, Aoys E, Moss G S. Total parenteral nutrition promotes bacterial translocation from the gut. Surgery 1988; 104:185-190).
TPN supplemented with glutamine restored the normal SIgA levels in bile. Glutamine supplementation was also beneficial in models of mucosal damage (radiation, methotrexate) by significantly decreasing the mortality of animals and accelerating mucosal recovery. These data emphasize the importance of glutamine for two distinct functions of the small intestine: (I) nutrient absorption and (II) the mucosal defense (Burke D J, Alverdy J C, Aoys E, Moss G. Glutamine-supplemented total parenteral nutrition improves gut immune function. Arch Surg 1989; 124:1396-1399; Fox A D, Kripke S A, De Paula J, Berman J M, Settle R G, Rombeau J L. Effect of a glutamine-supplemented enteral diet on methotrexate-induced enterocolitis. J Parent Enteral Nutr 1988;12:325-331; Klimberg V S, Souba W W, Dolson D, Copeland E M. Oral glutamine supports crypt cell turnover and accelerates intestinal healing following abdominal radiation. JPEN 1989; 115:38 (abstract)).
Effects of Glutamine on the Small Intestinal Mucosa
Glutamine increases the number of mitoses per crypt in animals fed a glutamine-supplemented elemental diet.
That implies that glutamine supports crypt cell turnover and leads to increased villous height. Burke et al demonstrated that the addition of glutamine to TPN resulted in the maintenance of normal levels of IgA (Barber A E, Jones W G, Minei J P, Moldawer L L, Fahey T J, Lowry S F, Shires G T. Composition and functional consequences of fiber and glutamine supplementation of enteral diets. Surg Forum 1989;40:15-16; Klimberg V S, Souba W W, Dolson D, Copeland E M. Oral glutamine supports crypt cell turnover and accelerates intestinal healing following abdominal radiation. JPEN 1989; 115:38 (abstract)). (Hwang T L, O'Dwyer S, Smith R J, Wilmore D W. Preservation of the small intestinal mucosa using glutamine supplemented parenteral nutrition. Surg Forum 1986; 37:56-58); Burke D J, Alverdy J C, Aoys E, Moss G. Glutamine-supplemented total parenteral nutrition improves gut immune function, 1989; 124:1396-1399).
Underlying mechanism of actions of glutamine on the intestinal mucosa are not known. Li et al reported an elevated concentration of glucagon in the portal vein of rats receiving glutamine containing TPN. Glucagon has an important role in the regulation of glutaminase. O'Dwyer suggested that the trophic effect of glutamine may be related to its secretagogue action, stimulating enteroglucagon secretion (Li S, Nussbaum M S, McFadden D W, Zhang F-S, LaFrance R J, Dayal R, Fischer J E. Addition of L-glutamine to total parenteral nutrition and its effect on portal insulin and glucagon and the development of hepatic steatosis in rats. J Surg Res 1990; 48:421-426); Geer R J, Williams P E, Lairmore T, Abumrad N N. Glucagon: an important stimulator of gut and hepatic glutamine metabolism. Surg Forum 1987; 38:27-28); O'Dwyer S T, Smith R J, Hwang T L, Wilmore D W. Maintenance of small bowel mucosa with glutamine enriched parenteral nutrition. JPEN 1989; 13:579-585).
Glutamine and the Neonatal Intestine
Very little data are available concerning the role of glutamine in the developing intestine. Kimura demonstrated an increased glutamine oxidation in the bowel of newborn rats compared to adult animals, indicating a higher demand for glutamine during development (Kimura R E. Glutamine oxidation by developing rat small intestine. Pediatr Res 1987; 21:214-217).
The glutamate content of human milk protein is very high and it is the most abundant amino acid in a variety of milk proteins (casein, serum albumin, lactoferrin, IgA and a-lactalbumin). Glutamine, glutamic acid and taurine are the most abundant free amino acids in human milk. Presumably, the high glutamate and glutamine content is advantageous for the developing small intestine (Harzer G, Bindels J G. Main compositional criteria of human milk and their implications on nutrition in early infancy. In: New aspects of nutrition in pregnancy, infancy and prematurity. (ed. Xanthou M). Elsevier Science Publishers, Amsterdam, 1987, p. 83-94; Rassin D K. Protein requirements in neonate. In: Textbook of Gastroenterology and Nutrition in Infancy (ed. Lebenthal E.), Raven Press, Ltd., New York, 1989, pp. 281-292; Harzer D, Franzke V, Bindels J G. Human milk nonprotein nitrogen components: changing patterns of free amino acid and urea in the course of early lactation. Am J Clin Nutr 1984; 40:303-309.
Very recently, Neu et al. have studied glutamine metabolism in preterm infants and have shown that preterm infants can tolerate and utilize glutamine when provided orally (enterally). Examining the immunological functions, these authors reported a reduced HLA-DR(+) T cell population and a concomitant increase in nosocomial infection in preterm infants not receiving supplemental oral glutamine.
Bacterial Translocation
Bacterial translocation is defined as the passage of viable intestinal bacteria across the intestinal epithelial cell layer into the normally sterile extra intestinal tissues. The translocated bacteria are usually normal inhabitants of the lower part of the small intestine and the colon. Translocation of bacteria may occur both transcellularly and paracellularly (Alexander J W, Bryce S T, Babock G F et al. The process of microbial translocation. Ann Surg 1990;212:496-512).
The first step in the process of translocation is the traffic of bacteria across the epithelial cell (enterocyte) monolayer. Translocation of few bacteria is a normal process and the mucosal immune system (macrophages as first line of defense) along with the consequent immune activation prevent further translocation. Secretory immunoglobulins may prevent the attachment of the same bacteria into the mucosal surface.
In the absence of glutamine (TPN) expression of secretory IgA is decreased. The Golgi-apparatus plays an important role in secretory IgA production. The production of secretory component takes place in the rough endoplasmic reticulum and it needs further maturation in the Golgi apparatus.
The morphological changes in the Golgi-apparatus described by our group may explain the decreased SIgA production found in patients on TPN. Any decline in immune defense results in deeper invasion of bacteria, and they can be detected in the mesenteric lymph nodes, liver and spleen. (Horvath K, Jami M M, Hill I D, Papadimitriou J C, Magder L S, Chanasongcram S. Glutamine-free oral diet and the morphology and function of the rat small intestine. J Parent Ent Nutr 1996;20: 128-134; Burke D J, Alverdy J C, Aoys E, Moss G. Glutamine-supplemented total parenteral nutrition improves gut immune function. Arch Surg 1989; 124:1396-1399; Brandtzaeg P, Halstensen T S, Kett K, Kraj.sub.-- i P, Kvale D, Rognum TO, Scott H, Sollid L M. Immunobiology and immunopathology of human gut mucosa: humoral immunity and intraepithelail lymphocytes. Gastroenterology 1989; 97:1562-84).
Necrotizing Enterocolitis in Premature Infants
NEC is the most serious gastrointestinal disorder of premature infants and one of the leading causes of death in neonatal intensive care units (NICU). It is the most common surgical emergency in the newborn period and the second leading cause of morbidity and mortality in the preterm population. The incidence of NEC in selected studies has ranged from fewer than 1% to as many as 5% of NICU admissions. A recent multicenter study of 2681 infants weighing 501-1500 grams reported that proven NEC (Bell Stage 2-3) occurred in 10.1% and suspected NEC (Bell Stage 1) in a further 17.2% of the cohort; mortality was 54% in infants with severe (Stage 3) NEC.
Those data indicate that NEC is a major public health problem in neonates: given the .about.4 million births/year in the United States, NEC would be expected to develop in 1200-9600 infants, of whom between 9-28% will die as a result of their disease. Earlier studies indicated a mortality of 10-55% in premature infants. Survivors of NEC can also have considerable long-term morbidity resulting from their disease, including short-gut syndrome, failure-to-thrive, intestinal stricture, and the need for repeated surgery.
Clinical Significance
Several studies have demonstrated the beneficial effect of glutamine in the intestine and especially on enterocytes (Barber A E, Jones W G, Minei J P, Moldawer L L, Fahey T J, Lowry S F, Shires G T. Composition and functional consequences of fiber and glutamine supplementation of enteral diets. Surg Forum 1989;40:15-16; Klimberg V S, Souba W W, Dolson D, Copeland E M). Oral glutamine supports crypt cell turnover and accelerates intestinal healing following abdominal radiation. JPEN 1989; 115:38 (abstract); Souba W W, Klimberg V S, Hautamaki R D, Meddenhall W H, Bova F C, Howard R J, Bland K I, Copeland E M. Oral glutamine reduces bacterial translocation following abdominal radiation. J Surg Res 1990; 48:1-5), however, the molecular mechanism of these effects has not been clarified. (Hwang T L, O'Dwyer S, Smith R J, Wilmore D W. Preservation of the small intestinal mucosa using glutamine supplemented parenteral nutrition. Surg Forum 1986; 37:56-58).
In different experimental animal models it has been shown that endotoxemia, ischemia, hemorrhagic shock can cause bacterial translocation. (Deitch E A, Berg R D, Specian R. Endotoxin promotes the translocation of bacteria from the gut. Arch Surg 1987; 122:185; Redan J A, Rush B F, Lysz T W, Smith S, Machiedo G W. Organ distribution of gut-derived bacteria caused by bowel manipulation or ischemia. Am J Surg 1990;159:85-89; Deitch E A, Bridges W, Baker J, Ma J W, Ma I, Grisham M B, Grenger D N, Specian R D, Berg R D). Hemorrhagic shock induced bacterial translocation is reduced by xanthine oxidase inhibition or inactivation. Surgery 1988; 104:191), burn and infection, chemotherapy, abdominal radiation. (Jones II W G, Minei J P, Barber A E, Raybern J, Fahey III T J, Shires III G T, Shires G T. Bacterial translocation and intestinal atrophy after injury and burn wound sepsis. Ann Surg 1990;211:399; Berg R D. Bacterial translocation from the gastrointestinal tract of mice receiving immunosuppressive chemotherapeutic agents. Curr Microbiol 1983; 8: 285-289; Fox A D, Kripke S A, De Paula J, Berman J M, Settle R G, Rombeau J L. Effect of a glutamine-supplemented enteral diet on methotrexate-induced enterocolitis. J Parent Enteral Nutr 1988;12:325-331; Guzman-Stein G, Bonsack M, Liberty J, Delaney J P). Abdominal radiation causes bacterial translocation, total parenteral nutrition, total parenteral nutrition plus narcotics and impaired intestinal motility can cause bacterial translocation to mesenteric lymph nodes, abdominal cavity, liver, spleen and blood resulting in septicemia and death. The cause of such increased bacterial translocation has not been defined. (J Surg Res 1989;46:104; Souba W W, Klimberg V S, Hautamaki R D, Meddenhall W H, Bova F C, Howard R J, Bland K I, Copeland E M. Oral glutamine reduces bacterial translocation following abdominal radiation. J Surg Res 1990; 48:1-5; Alverdy J C, Aoys E, Moss G S. Total parenteral nutrition promotes bacterial translocation from the gut. Surgery 1988; 104:185-190; Kueppers P M, Miller T A, Chen C Y K et al . . . Effect of total parenteral nutrition plus morphine on bacterial translocation in rats. Ann Surg 1993;217:286-292).
Prospective randomized clinical trials have documented that the incidence of major infectious complications is less in enterally fed burn and trauma, patients than in comparable patients fed parenterally. (Alexander J W, MacMillan J C, Stinnet J D. et al. Beneficial effect of aggressive protein feeding in severely burned children. Ann Surg 1980; 192:505-517; Kudsk K A, Groce M A, Fabian T C et al. Enteral versus parenteral feeding: Effects on septic morbidity after blunt and penetrating abdominal trauma. Ann Surg 1992; 215:503-513; Moore F A., Moore E E, Jones T N et al. TEN versus TPN following major torso trauma: reduced septic morbidity. J Trauma 1989; 29:916-923).
Sedman et al examined 242 general surgical patients for bacterial translocation during surgery. 10.3% of the patients had translocation detected on the intestinal serosa or in the mesenteric lymph nodes. Intestinal obstruction and inflammatory bowel disease were predisposing factors for translocation, however, 5% of patients without these conditions had translocation. The development of postoperative septic complications was twice as common in patients with translocation as those without it. (Sedman P C, Macfie J, Sagar P, Mitchell C J, May J, Mancey-Jones B, Johnstone D. The prevalence of gut translocation in humans. Gastroenterology 1994;107:643-649).